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The world’s largest nuclear fusion machine, currently being built in France, is unlikely to produce more energy than it consumes until the early 2030s, warned the UK’s head of fusion research this week. That is five years later than planned – by which time China could be ahead of everyone.

Nuclear fusion involves heating a plasma of hydrogen isotopes so that they fuse into helium, releasing a large amount of energy in the process. Many physicists see it as the holy grail for producing cheap zero-carbon energy. But initiating the fusion reactions requires temperatures 10 times as hot as the core of the sun. And decades of experiments have yet to produce self-sustaining fusion reactions – known as “burning plasma” – that generate the energy required to produce such temperatures.

“We are confident that it will,” Steven Cowley, director of the Culham Centre for Fusion Energy in Oxfordshire, told the science and technology committee of the UK’s House of Lords on Tuesday. But it is taking time and money.

Burning plasma

Constructing ITER has already cost three times as much as budgeted, and completion has slipped from 2016 to 2019, with the first plasma experiments the following year. Cowley told the committee: “ITER says 2020, but I believe the first plasma will be [generated] in 2025.” Burning plasma is unlikely before “the early 2030s”, he said. He likened the moment when burning plasma is achieved to the moment in the early 1940s when the first “critical” nuclear fission reactions were produced.

Only then will the international researchers, many of whom have been working together for decades, move on to building a new plant that could generate continuous power – the forerunner for what they hope will be commercial nuclear fusion by late in the century. “But the biggest investment now is in China,” says Cowley. China is a collaborator on ITER, along with the European Union, the US and others. But it is investing heavily in building its own reactor, the China Fusion Engineering Test Reactor, which will be bigger than ITER and may be finished by 2030, he said.

Cowley disclosed that some partners had discussed whether to continue collaboration with China or shut them out. “We decided to continue to collaborate.” Shutting China out “would only slow them down by a few months”, he told the Lords, who are investigating whether the UK government is getting value for money in its fusion investments. Fusion currently accounts for 14 per cent of UK government spending on energy research, Sharon Ellis of the Department for Business, Innovation and Skills told the committee.

Los Alamos National Laboratory (LANL), teamed with Hyper V Technologies and a multi-institutional team, will develop a plasma-liner driver formed by merging supersonic plasma jets produced by an array of coaxial plasma guns.

The key virtues of a plasma-liner driver, as noted by project leader Scott Hsu, are that it (1) has standoff, i.e., it completely avoids hardware destruction because the plasma guns are placed sufficiently far away (many meters in an eventual fusion reactor) from the region of fusion burn, and (2) it enables high implosion velocity (50–100 km/s) to overcome thermal transport rates inherent in desired targets.

This non-destructive approach may enable rapid, low cost research and development and, by avoiding replacement of solid components on every shot, may help lead to an economically attractive power reactor. This project will seek to demonstrate, for the first time, the formation of a small scale spherically imploding plasma liner in order to obtain critical data on plasma liner uniformity and ram pressure scaling. If successful, this concept will provide a versatile, high-implosion-velocity driver for intermediate fuel density magneto-inertial fusion that is potentially compatible with several plasma targets. These experiments will be conducted on the existing Plasma Liner Experiment (PLX) facility at Technical Area 35 at Los Alamos.

Stabilized Liner Compressor (SLC) for Low-Cost Fusion

NumerEx, LLC, teamed with the National High Magnetic Field Laboratory in Los Alamos, NM, will develop the Stabilized Liner Compressor (SLC) concept in which a rotating, liquid metal liner is imploded by high-pressure gas.

The Stabilized Liner Compressor (SLC) is a system that uses high-pressure gas and a free-piston to implode a liquid metal liner onto trapped magnetic flux in order to achieve controlled fusion at very high magnetic fields (~100 T).

“The SLC project provides an opportunity to leverage advances in materials in a new era of computation capabilities while developing a revolutionary high magnetic field capability with a distinct purpose,” said Los Alamos project leader Chuck Mielke.

Free-piston drive and liner rotation avoid instabilities as the liner compresses and heats a plasma target. If successful, this concept could scale to an attractive fusion reactor with efficient energy recovery, and therefore a low required minimum fusion gain for net energy output. The SLC will address several challenges faced by practical fusion reactors. By surrounding the plasma target with a thick liquid liner, the SLC helps avoid materials degradation associated with a solid plasma-facing first wall. In addition, with an appropriately chosen liner material, the SLC can simultaneously provide a breeding blanket to create more tritium fuel, allow efficient heat transport out of the reactor, and shield solid components of the reactor from high-energy neutrons.

“We recognized back at the Naval Research Laboratory in the 1970s that there may exist an optimum regime for controlled fusion at much higher magnetic fields than used by the mainline magnetic fusion program, but at much lower power density than required for laser fusion. The resulting power reactor and the necessary experimental prototypes need the repetitive, stabilized operation at megagauss field-levels offered by SLC,” said Peter J. Turchi, Los Alamos Guest Scientist and Senior Consultant to NumerEx LLC.

Prototype Tools to Establish the Viability of the Adiabatic Heating and Compression Mechanisms Required for Magnetized Target Fusion

Caltech, in coordination with Los Alamos National Laboratory, will investigate collisions of plasma jets and targets over a wide range of parameters to characterize the scaling of adiabatic heating and compression of liner-driven magnetized target fusion plasmas.

“Los Alamos will provide plasma physics modeling of the experiments to be carried out at Caltech to understand the critical processes during the plasma-cloud interactions,” said Hui Li, the lead Los Alamos scientist on the project.

The team will propel fast, magnetized plasma jets into stationary heavy gases or metal walls. The resulting collision is equivalent to a fast heavy gas or metal liner impacting a stationary magnetized target in a shifted reference frame and allows the non-destructive and rapid investigation of physical phenomena and scaling laws governing the degree of adiabaticity of liner implosions. This study will provide critical information on the interactions and limitations for a variety of possible driver and plasma target combinations being developed across the ALPHA program portfolio.

Los Alamos enhances national security by ensuring the safety and reliability of the U.S. nuclear stockpile, developing technologies to reduce threats from weapons of mass destruction, and solving problems related to energy, environment, infrastructure, health, and global security concerns.

Operated by Los Alamos National Security, LLC for the U.S. Dept. of Energy’s NNSA

UW researchers will attempt to create a self-sustained and controlled fusion reaction with a scaled-up version of this Z-pinch device. University of Washington

Producing reliable fusion energy — the same process that powers the sun — has long been a holy grail of scientists here on Earth. It releases no greenhouse gases, can be fueled by elements found in seawater and produces no long-lived nuclear waste.

The basic mechanism — getting two nuclei that want nothing to do with each other to fuse — is also difficult enough that there’s no danger of a runaway chain reaction. In fact, scientists so far have struggled to create self-sustained controlled fusion reactions that produce more energy than they consume.

University of Washington researchers have spent two decades developing a novel way to provide plasma stability that’s critical to achieving fusion. With a $5.3 million U.S. Department of Energy grant announced in May, they will partner with Lawrence Livermore National Laboratory to scale up their “Sheared Flow Stabilized Z-Pinch” device in the hopes of achieving a sustainable fusion reaction that might one day power homes or propel spaceships far beyond current limitations.

“Fusion energy is the ultimate energy source. It’s free of greenhouses gases, and it also has the potential to be a very robust source without the reliability problems of wind and solar,” said UW professor of aeronautics and astronautics Uri Shumlak, who collaborated with UW electrical engineering professor Brian Nelson to develop the device.

It will be the first time that UW researchers have built a fusion device on campus, Shumlak said.

“Researchers generally don’t achieve adequate plasma conditions to produce significant fusion reactions,” said Shumlak. “Our project will be a proof-of-principle experiment, and just showing that the sheared flow stabilized Z-pinch approach scales to higher powers is going to be really exciting.”

Fusion is the opposite of fission, which splits heavier atoms apart and produces the energy that powers commercial nuclear reactors. Fusing smaller atoms together can yield even greater amounts of energy but does not typically produce unpredictable radioactive isotopes or long-lived radioactive waste. The UW’s experiments, for instance, would be fueled by stable and harmless isotopes of hydrogen that are widely available in nature.

The fusion process does produce neutrons that can pose hazards if not properly controlled, but their behavior is well understood. Neutron therapy, for instance, is used to treat certain types of cancers. UW researchers will closely monitor emissions and follow well-established protocols to ensure those levels pose no risks.

The Sheared Flow Stabilized Z-Pinch has a simple, linear configuration and uses sheared axial flows to prevent plasma instabilities from growing. The concept is similar to cars in the center lane of the highway being prevented from changing lanes by faster moving traffic on either side. University of Washington

Most university research has focused on the basic science involved in creating, confining and stabilizing plasma, which is a basic ingredient for fusion. Often called the “fourth state of matter,” plasma forms when a gas is so superheated that electrons are ripped apart from an atom’s nucleus.

Applying enough energy to this swirl of negatively and positively charged particles can induce the nuclei to fuse. Under the right conditions — which have proved devilishly difficult to create outside of stars like the sun — this process gives off more energy than it consumes.

One problem is that simply creating plasma requires such high temperatures — typically greater than 200,000 degrees Fahrenheit — that nothing material can contain it without disintegrating or melting. One approach to fusion research uses magnetic fields, often generated by gigantic coils that are many stories high, to contain the plasma.

UW Aeronautics and Astronautics professor Uri Shumlak and student Bonghan Kim work on an earlier prototype of the Z-pinch device.University of Washington

The UW researchers have used a Z-pinch, which is a geometrically simple and elegant approach to fusion that uses an electric current to magnetically confine, compress and heat a long cylinder of flowing plasma. It requires no magnetic coils, which means that the device could be much smaller, cheaper and more versatile than some of the massive fusion machines under development today.

One historic problem with the z-pinch is that the interface between the plasma and the magnetic fields is unstable. They essentially try to invert and trade places, just like a layer of water laid on top of a layer of oil will try to flip. The UW researchers have developed an accelerator that manipulates the properties of the plasma itself to create more stable conditions — at least at lower temperatures.

“Sheared flow stabilization uses plasma moving at different speeds in different places to prevent plasma instabilities from growing,” said Shumlak. “It’s something like cars in the center lane of a freeway that are prevented from changing lanes by higher speed traffic on both sides.”

The 3-year DOE grant will enable the researchers to test if the concept still works at temperatures high enough to create fusion conditions, about 20 million degrees F. They will need to increase the amount of energy that has been injected into the Z-pinches they’ve built to date by more than tenfold.

The UW researchers are collaborating with scientists Harry McLean and Andréa Schmidt of Lawrence Livermore National Laboratory, who provide expertise in designing the higher-energy power supplies and detecting neutrons as evidence that atoms have fused.

The team plans to build a new Z-pinch device at the UW by summer of 2016 and run its first fusion tests in 2017.

“Essentially, we need to determine if we can scale the sheared flow stabilized Z-pinch,” said Nelson. “As we go to higher Z-pinch currents by injecting more energy, does it still stabilize and compress the plasma? Or, put more simply, does the concept still work?”

Funding for the project comes from the U.S. Department of Energy’s Advanced Research Projects Agency-Energy (ARPA-E).

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Researchers at the U.S. Department of Energy’s Princeton Plasma Physics Laboratory (PPPL) have for the first time simulated the formation of structures called “plasmoids” during Coaxial Helicity Injection (CHI), a process that could simplify the design of fusion facilities known as tokamaks. The findings, reported in the journal Physical Review Letters, involve the formation of plasmoids in the hot, charged plasma gas that fuels fusion reactions. These round structures carry current that could eliminate the need for solenoids – large magnetic coils that wind down the center of today’s tokamaks – to initiate the plasma and complete the magnetic field that confines the hot gas.

PPPL Tokamak

“Understanding this behavior will help us produce plasmas that undergo fusion reactions indefinitely,” said Fatima Ebrahimi, a physicist at both Princeton University and PPPL, and the paper’s lead author.

Ebrahimi ran a computer simulation that modeled the behavior of plasma and the formation of plasmoids in three dimensions thoughout a tokamak’s vacuum vessel. This marked the first time researchers had modeled plasmoids in conditions that closely mimicked those within an actual tokamak. All previous simulations had modeled only a thin slice of the plasma – a simplified picture that could fail to capture the full range of plasma behavior.

These images also showed plasmoid-like structures, confirming the simulation and giving the research breakthrough significance, since it revealed the existence of plasmoids in an environment in which they had never been seen before. “These findings are in a whole different league from previous ones,” said Roger Raman, leader for the Coaxial Helicity Injection Research program on NSTX and a coauthor of the paper.

The findings may provide theoretical support for the design of a new kind of tokamak with no need for a large solenoid to complete the magnetic field. Solenoids create magnetic fields when electric current courses through them in relatively short pulses. Today’s conventional tokamaks, which are shaped like a donut, and spherical tokamaks, which are shaped like a cored apple, both employ solenoids. But future tokamaks will need to operate in a constant or steady state for weeks or months at a time. Moreover, the space in which the solenoid fits – the hole in the middle of the doughnut-shaped tokamak – is relatively small and limits the size and strength of the solenoid.

A clear understanding of plasmoid formation could thus lead to a more efficient method of creating and maintaining a plasma through transient Coaxial Helicity Injection. This method, originally developed at the University of Washington, could dispense with a solenoid entirely and would work like this:

Researchers first inject open magnetic field lines into the vessel from the bottom of the vacuum chamber. As researchers drive electric current along those magnetic lines, the lines snap closed and form the plasmoids, much like soap bubbles being blown out of a sheet of soapy film.
The many plasmoids would then merge to form one giant plasmoid that could fill the vacuum chamber.
The magnetic field within this giant plasmoid would induce a current in the plasma to keep the gas tightly in place. “In principle, CHI could fundamentally change how tokamaks are built in the future,” says Raman.

Understanding how the magnetic lines in plasmoids snap closed could also help solar physicists decode the workings of the sun. Huge magnetic lines regularly loop off the surface of the star, bringing the sun’s hot plasma with them. These lines sometimes snap together to form a plasmoid-like mass that can interfere with communications satellites when it collides with the magnetic field that surrounds the Earth.

While Ebrahimi’s findings are promising, she stresses that much more is to come. PPPL’s National Spherical Torus Experiment-Upgrade (NSTX-U) will provide a more powerful platform for studying plasmoids when it begins operating this year, making Ebrahimi’s research “only the beginning of even more exciting work that will be done on PPPL equipment,” she said.

Princeton Plasma Physics Laboratory is a U.S. Department of Energy national laboratory managed by Princeton University. PPPL, on Princeton University’s Forrestal Campus in Plainsboro, N.J., is devoted to creating new knowledge about the physics of plasmas — ultra-hot, charged gases — and to developing practical solutions for the creation of fusion energy. Results of PPPL research have ranged from a portable nuclear materials detector for anti-terrorist use to universally employed computer codes for analyzing and predicting the outcome of fusion experiments. The Laboratory is managed by the University for the U.S. Department of Energy’s Office of Science, which is the largest single supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.

Scientists from General Atomics and the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) have discovered a phenomenon that helps them to improve fusion plasmas, a finding that may quicken the development of fusion energy. Together with a team of researchers from across the United States, the scientists found that when they injected tiny grains of lithium into a plasma undergoing a particular kind of turbulence then, under the right conditions, the temperature and pressure rose dramatically. High heat and pressure are crucial to fusion, a process in which atomic nuclei – or ions – smash together and release energy — making even a brief rise in pressure of great importance for the development of fusion energy.

“These findings might be a step towards creating our ultimate goal of steady-state fusion, which would last not just for milliseconds, but indefinitely,” said Tom Osborne, a physicist at General Atomics and lead author of the paper. This work was supported by the DOE Office of Science.

The scientists used a device developed at PPPL to inject grains of lithium measuring some 45 millionths of a meter in diameter into a plasma in the DIII-D National Fusion Facility – or tokamak – that General Atomics operates for DOE in San Diego.

DIII-D National Fusion Facility

When the lithium was injected while the plasma was relatively calm, the plasma remained basically unaltered. Yet as reported this month in a paper in Nuclear Fusion, when the plasma was undergoing a kind of turbulence known as a “bursty chirping mode,” the injection of lithium doubled the pressure at the outer edge of the plasma. In addition, the length of time that the plasma remained at high pressure rose by more than a factor of 10.

Experiments have sustained this enhanced state for up to one-third of a second. A key scientific objective will be to extend this enhanced performance for the full duration of a plasma discharge.

Physicists have long known that adding lithium to a fusion plasma increases its performance. The new findings surprised researchers, however, since the small amount of lithium raised the plasma’s temperature and pressure more than had been expected.

These results “could represent the birth of a new tool for influencing or perhaps controlling tokamak edge physics,” said Dennis Mansfield, a physicist at PPPL and a coauthor of the paper who helped develop the injection device called a “lithium dropper.” Also working on the experiments were researchers from Lawrence Livermore National Laboratory, Oak Ridge National Laboratory, the University of Wisconsin-Madison and the University of California-San Diego.

Conditions at the edge of the plasma have a profound effect on the superhot core of the plasma where fusion reactions take place. Increasing pressure at the edge region raises the pressure of the plasma as a whole. And the greater the plasma pressure, the more suitable conditions are for fusion reactions. “Making small changes at the plasma’s edge lets us increase the pressure further within the plasma,” said Rajesh Maingi, manager of edge physics and plasma-facing components at PPPL and a coauthor of the paper.

Further experiments will test whether the lithium’s interaction with the bursty chirping modes — so-called because the turbulence occurs in pulses and involves sudden changes in pitch — caused the unexpectedly strong overall effect.

Princeton Plasma Physics Laboratory is a U.S. Department of Energy national laboratory managed by Princeton University. PPPL, on Princeton University’s Forrestal Campus in Plainsboro, N.J., is devoted to creating new knowledge about the physics of plasmas — ultra-hot, charged gases — and to developing practical solutions for the creation of fusion energy. Results of PPPL research have ranged from a portable nuclear materials detector for anti-terrorist use to universally employed computer codes for analyzing and predicting the outcome of fusion experiments. The Laboratory is managed by the University for the U.S. Department of Energy’s Office of Science, which is the largest single supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.

ITER, the international fusion reactor being built in France, will stand 10 stories tall, weigh three times as much as the Eiffel Tower, and cost its seven international partners $18 billion or more. The result of decades of planning, ITER will not produce fusion energy until 2027 at the earliest. And it will be decades before an ITER-like plant pumps electricity into the grid. Surely there is a quicker and cheaper route to fusion energy.

Fusion enthusiasts have a slew of schemes for achieving the starlike temperatures or crushing pressures needed to get hydrogen nuclei to come together in an energy-spawning union. Some are mainstream, such as lasers, some unorthodox. Yet the doughnut-shaped vessels called tokamaks, designed to cage a superheated plasma using magnetic fields, remain the leading fusion strategy and are the basis of ITER. Even among tokamaks, however, a nimbler alternative has emerged: a spherical tokamak.

Imagine the doughnut shape of a conventional tokamak plumped up into a shape more like a cored apple. That simple change, say the idea’s advocates, could open the way to a fusion power plant that would match ITER’s promise, without the massive scale. “The aim is to make tokamaks smaller, cheaper, and faster—to reduce the eventual cost of electricity,” says Ian Chapman, head of tokamak science at the Culham Centre for Fusion Energy in Abingdon, U.K.

Culham is one of two labs about to give these portly tokamaks a major test. The world’s two front-rank machines—the National Spherical Torus Experiment (NSTX) at the Princeton Plasma Physics Laboratory in New Jersey and the Mega Amp Spherical Tokamak (MAST) in Culham—are both being upgraded with stronger magnets and more powerful heating systems. Soon they will switch on and heat hydrogen to temperatures much closer to those needed for generating fusion energy. If they perform well, then the next major tokamak to be built—a machine that would run in parallel with ITER and test technology for commercial reactors—will likely be a spherical tokamak.

NSTX

MAST

A small company spun off from Culham is even making a long-shot bet that it can have a spherical tokamak reactor capable of generating more energy than it consumes—one of ITER’s goals—up and running within the decade. If it succeeds, spherical tokamaks could change the shape of fusion’s future. “It’s going to be exciting,” says Howard Wilson, director of the York Plasma Institute at the University of York in the United Kingdom. “Spherical tokamaks are the new kids on the block. But there are still important questions we’re trying to get to the bottom of.”

TOKAMAKS ARE AN INGENIOUS WAY to cage one of the most unruly substances humans have ever grappled with: plasma hot enough to sustain fusion. To get nuclei to slam together and fuse, fusion reactors must reach temperatures 10 times hotter than the core of the sun, about 150 million degrees Celsius. The result is a tenuous ionized gas that would vaporize any material it touches—and yet must be held in place long enough for fusion to generate useful amounts of energy.

Tokamaks attempt this seemingly impossible task using magnets, which can hold and manipulate plasma because it is made of charged particles. A complex set of electromagnets encircle the doughnut-shaped vessel, some horizontal and some vertical, while one tightly wound coil of wire, called a solenoid, runs down the doughnut hole. Their combined magnetic field squeezes the plasma toward the center of the tube and drives it around the ring while also twisting in a slow corkscrew motion.

But plasma is not easy to master. Confining it is like trying to squeeze a balloon with your hands: It likes to bulge out between your fingers. The hotter a plasma gets, the more the magnetically confined gas bulges and wriggles and tries to escape. Much of the past 60 years of fusion research has focused on how to control plasma.

Generating and maintaining enough heat for fusion has been another challenge. Friction generated as the plasma surges around the tokamak supplies some of the heat, but modern tokamaks also beam in microwaves and high-energy particles. As fast as the heat is supplied, it bleeds away, as the hottest, fastest moving particles in the turbulent plasma swirl away from the hot core toward the cooler edge. “Any confinement system is going to be slightly leaky and will lose particles,” Wilson says.

Studies of tokamaks of different sizes and configurations have always pointed to the same message: To contain a plasma and keep it hot, bigger is better. In a bigger volume, hot particles have to travel farther to escape. Today’s biggest tokamak, the 8-meter-wide Joint European Torus (JET) at Culham, set a record for fusion energy in 1997, generating 16 megawatts for a few seconds.

JET

(That was still slightly less than the heating power pumped into the plasma.) For most of the fusion community, ITER is the logical next step. It is expected to be the first machine to achieve energy gain—more fusion energy out than heating power in.

In the 1980s, a team at Oak Ridge National Laboratory in Tennessee explored how a simple shape change could affect tokamak performance. They focused on the aspect ratio—the radius of the whole tokamak compared to the radius of the vacuum tube. (A Hula-Hoop has a very high aspect ratio, a bagel a lower one.) Their calculations suggested that making the aspect ratio very low, so that the tokamak was essentially a sphere with narrow hole through the middle, could have many advantages.

Near a spherical tokamak’s central hole, the Oak Ridge researchers predicted, particles would enjoy unusual stability. Instead of corkscrewing lazily around the tube as in a conventional tokamak, the magnetic field lines wind tightly around the central column, holding particles there for extended periods before they return to the outside surface. The D-shaped cross section of the plasma would also help suppress turbulence, improving energy confinement. And they reckoned that the new shape would use magnetic fields more efficiently—achieving more plasma pressure for a given magnetic pressure, a ratio known as beta. Higher beta means more bang for your magnetic buck. “The general idea of spherical tokamaks was to produce electricity on a smaller scale, and more cheaply,” Culham’s Chapman says.

But such a design posed a practical problem. The narrow central hole in a spherical tokamak didn’t leave enough room for the equipment that needs to fit there: part of each vertical magnet plus the central solenoid. In 1984, Martin Peng of Oak Ridge came up with an elegant, space-saving solution: replace the multitude of vertical ring magnets with C-shaped rings that share a single conductor down the center of the reactor (see graphic, below).

JAMES PROVOST

U.S. fusion funding was in short supply at that time, so Oak Ridge could not build a spherical machine to test Peng’s design. A few labs overseas converted some small devices designed for other purposes into spherical tokamaks, but the first true example was built at the Culham lab in 1990. “It was put together on a shoestring with parts from other machines,” Chapman says. Known as the Small Tight Aspect Ratio Tokamak (START), the device soon achieved a beta of 40%, more than three times that of any conventional tokamak.

It also bested traditional machines in terms of stability. “It smashed the world record at the time,” Chapman says. “People got more interested.” Other labs rushed to build small spherical tokamaks, some in countries not known for their fusion research, including Australia, Brazil, Egypt, Kazakhstan, Pakistan, and Turkey.

The next question, Chapman says, was “can we build a bigger machine and get similar performance?” Princeton and Culham’s machines were meant to answer that question. Completed in 1999, NSTX and MAST both hold plasmas about 3 meters across, roughly three times bigger than START’s but a third the size of JET’s. The performance of the pair showed that START wasn’t a one-off: again they achieved a beta of about 40%, reduced instabilities, and good confinement.

Now, both machines are moving to the next stage: more heating power to make a hotter plasma and stronger magnets to hold it in place. MAST is now in pieces, the empty vacuum vessel looking like a giant tin can adorned with portholes, while its €30 million worth of new magnets, pumps, power supplies, and heating systems are prepared. At Princeton, technicians are putting the finishing touches to a similar $94 million upgrade of NSTX’s magnets and neutral beam heating. Like most experimental tokamaks, the two machines are not aiming to produce lots of energy, just learning how to control and confine plasma under fusionlike conditions. “It’s a big step,” Chapman says. “NSTX-U will have really high injected power in a small plasma volume. Can you control that plasma? This is a necessary step before you could make a spherical tokamak power plant.”

The upgraded machines will each have a different emphasis. NSTX-U, with the greater heating power, will focus on controlling instabilities and improving confinement when it restarts this summer.

NSTX-U

“If we can get reasonable beta values, [NSTXU] will reach plasma [properties] similar to conventional tokamaks,” says NSTX chief Masayuki Ono. MAST-Upgrade, due to fire up in 2017, will address a different problem: capturing the fusion energy that would build up in a full-scale plant.

Fusion reactions generate most of their energy in the form of high-energy neutrons, which, being neutral, are immune to magnetic fields and can shoot straight out of the reactor. In a future power plant, a neutron-absorbing material will capture them, converting their energy to heat that will drive a steam turbine and generate electricity. But 20% of the reaction energy heats the plasma directly and must somehow be tapped. Modern tokamaks remove heat by shaping the magnetic field into a kind of exhaust pipe, called a divertor, which siphons off some of the outermost layer of plasma and pipes it away. But fusion heat will build up even faster in a spherical tokamak because of its compact size. MAST-Upgrade has a flexible magnet system so that researchers can try out various divertor designs, looking for one that can cope with the heat.

Researchers know from experience that when a tokamak steps up in size or power, plasma can start misbehaving in new ways. “We need MAST and NSTX to make sure there are no surprises at low aspect ratio,” says Dennis Whyte, director of the Plasma Science and Fusion Center at the Massachusetts Institute of Technology in Cambridge. Once NSTX and MAST have shown what they are capable of, Wilson says, “we can pin down what a [power-producing] spherical tokamak will look like. If confinement is good, we can make a very compact machine, around MAST size.”

BUT GENERATING ELECTRICITY isn’t the only potential goal. The fusion community will soon have to build a reactor to test how components for a future power plant would hold up under years of bombardment by high-energy neutrons. That’s the goal of a proposed machine known in Europe as the Component Test Facility (CTF), which could run stably around the clock, generating as much heat from fusion as it consumes. A CTF is “absolutely necessary,” Chapman says. “It’s very important to test materials to make reactors out of.” The design of CTF hasn’t been settled, but spherical tokamak proponents argue their design offers an efficient route to such a testbed—one that “would be relatively compact and cheap to build and run,” Ono says.

With ITER construction consuming much of the world’s fusion budget, that promise won’t be tested anytime soon. But one company hopes to go from a standing start to a small power-producing spherical tokamak in a decade. In 2009, a couple of researchers from Culham created a spinoff company—Tokamak Solutions—to build small spherical tokamaks as neutron sources for research. Later, one of the company’s suppliers showed them a new multilayered conducting tape, made with the high-temperature superconductor yttrium-barium-copper-oxide, that promised a major performance boost.

Lacking electrical resistance, superconductors can be wound into electromagnets that produce much stronger fields than conventional copper magnets. ITER will use low-temperature superconductors for its magnets, but they require massive and expensive cooling. High-temperature materials are cheaper to use but were thought to be unable to withstand the strong magnetic fields around a tokamak—until the new superconducting tape came along. The company changed direction, was renamed Tokamak Energy, and is now testing a first-generation superconducting spherical tokamak no taller than a person.

Superconductors allow a tokamak to confine a plasma for longer. Whereas NSTX and MAST can run for only a few seconds, the team at Tokamak Energy this year ran their machine—albeit at low temperature and pressure—for more than 15 minutes. In the coming months, they will attempt a 24-hour pulse—smashing the tokamak record of slightly over 5 hours.

Next year, the company will put together a slightly larger machine able to produce twice the magnetic field of NSTX-U. The next step—investors permitting—will be a machine slightly smaller than Princeton’s but with three times the magnetic field. Company CEO David Kingham thinks that will be enough to beat ITER to the prize: a net gain of energy. “We want to get fusion gain in 5 years. That’s the challenge,” he says.

“It’s a high-risk approach,” Wilson says. “They’re buying their lottery ticket. If they win, it’ll be great. If they don’t, they’ll likely disappear. Even if it doesn’t work, we’ll learn from it; it will accelerate the fusion program.”

It’s a spirit familiar to everyone trying to reshape the future of fusion.

The gold standard for analyzing the behavior of fusion plasmas may have just gotten better. Mario Podestà, a staff physicist at the U.S. Department of Energy’s Princeton Plasma Physics Laboratory (PPPL), has updated the worldwide computer program known as TRANSP to better simulate the interaction between energetic particles and instabilities – disturbances in plasma that can halt fusion reactions. The program’s updates, reported in the journal Nuclear Fusion, could lead to improved capability for predicting the effects of some types of instabilities in future facilities such as ITER, the international experiment under construction in France to demonstrate the feasibility of fusion power.

ITER Tokamak

Podestà and co-authors saw a need for better modeling techniques when they noticed that while TRANSP could accurately simulate an entire plasma discharge, the code wasn’t able to represent properly the interaction between energetic particles and instabilities. The reason was that TRANSP, which PPPL developed and has regularly updated, treated all fast-moving particles within the plasma the same way. Those instabilities, however, can affect different parts of the plasma in different ways through so-called “resonant processes.”

The authors first figured out how to condense information from other codes that do model the interaction accurately – albeit over short time periods – so that TRANSP could incorporate that information into its simulations. Podestà then teamed up with TRANSP developer Marina Gorelenkova at PPPL to update a TRANSP module called NUBEAM to enable it to make sense of this condensed data. “Once validated, the updated module will provide a better and more accurate way to compute the transport of energetic particles,” said Podestà. “Having a more accurate description of the particle interactions with instabilities can improve the fidelity of the program’s simulations.”

Schematic of NSTX tokamak at PPPL with a cross-section showing perturbations of the plasma profiles caused by instabilities. Without instabilities, energetic particles would follow closed trajectories and stay confined inside the plasma (blue orbit). With instabilities, trajectories can be modified and some particles may eventually be pushed out of the plasma boundary and lost (red orbit). Credit: Mario Podestà

Fast-moving particles, which result from neutral beam injection into tokamak plasmas, cause the instabilities that the updated code models. These particles begin their lives with a neutral charge but turn into negatively charged electrons and positively charged ions – or atomic nuclei – inside the plasma. This scheme is used to heat the plasma and to drive part of the electric current that completes the magnetic field confining the plasma.

PPPL Tokamak

The improved simulation tool may have applications for ITER, which will use fusion end-products called alpha particles to sustain high plasma temperatures. But just like the neutral beam particles in current-day-tokamaks, alpha particles could cause instabilities that degrade the yield of fusion reactions. “In present research devices, only very few, if any, alpha particles are generated,” said Podestà. “So we have to study and understand the effects of energetic ions from neutral beam injectors as a proxy for what will happen in future fusion reactors.”

PPPL, on Princeton University’s Forrestal Campus in Plainsboro, N.J., is devoted to creating new knowledge about the physics of plasmas — ultra-hot, charged gases — and to developing practical solutions for the creation of fusion energy. Results of PPPL research have ranged from a portable nuclear materials detector for anti-terrorist use to universally employed computer codes for analyzing and predicting the outcome of fusion experiments. The Laboratory is managed by the University for the U.S. Department of Energy’s Office of Science, which is the largest single supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, visit science.energy.gov.

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Computer simulation of a cross-section of a DIII-D plasma responding to tiny magnetic fields. The left image models the response that suppressed the ELMs while the right image shows a response that was ineffective.

Researchers from General Atomics and the U.S. Department of Energy’s Princeton Plasma Physics Laboratory (PPPL) have made a major breakthrough in understanding how potentially damaging heat bursts inside a fusion reactor can be controlled. Scientists performed the experiments on the DIII-D National Fusion Facility, a tokamak operated by General Atomics in San Diego. The findings represent a key step in predicting how to control heat bursts in future fusion facilities including ITER, an international experiment under construction in France to demonstrate the feasibility of fusion energy. This work is supported by the DOE Office of Science (Fusion Energy Sciences).

The studies build upon previous work pioneered on DIII-D showing that these intense heat bursts – called “ELMs” for short – could be suppressed with tiny magnetic fields. These tiny fields cause the edge of the plasma to smoothly release heat, thereby avoiding the damaging heat bursts. But until now, scientists did not understand how these fields worked. “Many mysteries surrounded how the plasma distorts to suppress these heat bursts,” said Carlos Paz-Soldan, a General Atomics scientist and lead author of the first of the two papers that report the seminal findings back-to-back in the same issue of Physical Review Letters this week.

Paz-Soldan and a multi-institutional team of researchers found that tiny magnetic fields applied to the device can create two distinct kinds of response, rather than just one response as previously thought. The new response produces a ripple in the magnetic field near the plasma edge, allowing more heat to leak out at just the right rate to avert the intense heat bursts. Researchers applied the magnetic fields by running electrical current through coils around the plasma. Pickup coils then detected the plasma response, much as the microphone on a guitar picks up string vibrations.

The second result, led by PPPL scientist Raffi Nazikian, who heads the PPPL research team at DIII-D, identified the changes in the plasma that lead to the suppression of the large edge heat bursts or ELMs. The team found clear evidence that the plasma was deforming in just the way needed to allow the heat to slowly leak out. The measured magnetic distortions of the plasma edge indicated that the magnetic field was gently tearing in a narrow layer, a key prediction for how heat bursts can be prevented. “The configuration changes suddenly when the plasma is tapped in a certain way,” Nazikian said, “and it is this response that suppresses the ELMs.”

The work involved a multi-institutional team of researchers who for years have been working toward an understanding of this process. These researchers included people from General Atomics, PPPL, Oak Ridge National Laboratory, Columbia University, Australian National University, the University of California-San Diego, the University of Wisconsin-Madison, and several others.

The new results suggest further possibilities for tuning the magnetic fields to make ELM-control easier. These findings point the way to overcoming a persistent barrier to sustained fusion reactions. “The identification of the physical processes that lead to ELM suppression when applying a small 3D magnetic field to the inherently 2D tokamak field provides new confidence that such a technique can be optimized in eliminating ELMs in ITER and future fusion devices,” said Mickey Wade, the DIII-D program director.

The results further highlight the value of the long-term multi-institutional collaboration between General Atomics, PPPL and other institutions in DIII-D research. This collaboration, said Wade, “was instrumental in developing the best experiment possible, realizing the significance of the results, and carrying out the analysis that led to publication of these important findings.”

Researchers from the UK firm Tokamak Energy say that future fusion reactors could be made much smaller than previously envisaged – yet still deliver the same energy output. That claim is based on calculations showing that the fusion power gain – a measure of the ratio of the power from a fusion reactor to the power required to maintain the plasma in steady state – does not depend strongly on the size of the reactor. The company’s finding goes against conventional thinking, which says that a large power output is only possible by building bigger fusion reactors.

The largest fusion reactor currently under construction is the €16bn ITER facility in Cadarache, France.

This will weigh about 23,000 tonnes when completed in the coming decade and consist of a deuterium–tritium plasma held in a 60 m-tall, doughnut-shaped “tokamak”. ITER aims to produce a fusion power gain (Q) of 10, meaning that, in theory, the reactor will emit 10 times the power it expends by producing 500 MW from 50 MW of input power. While ITER has a “major” plasma radius of 6.21 m, it is thought that an actual future fusion power plant delivering power to the grid would need a 9 m radius to generate 1 GW.

Low power brings high performance

The new study, led by Alan Costley from Tokamak Energy, which builds compact tokamaks, shows that smaller, lower-power, and therefore lower-cost reactors could still deliver a value of Q similar to ITER. The work focused on a key parameter in determining plasma performance called the plasma “beta”, which is the ratio of the plasma pressure to the magnetic pressure. By using scaling expressions consistent with existing experiments, the researchers show that the power needed for high fusion performance can be three or four times lower than previously thought.

Combined with the finding on the size-dependence of Q, these results imply the possibility of building lower-power, smaller and cheaper pilot plants and reactors. “The consequence of beta-independent scaling is that tokamaks could be much smaller, but still have a high power gain,” David Kingham, Tokamak Energy chief executive, told Physics World.

The researchers propose that a reactor with a radius of just 1.35 m would be able to generate 180 MW, with a Q of 5. This would result in a reactor just 1/20th of the size of ITER. “Although there are still engineering challenges to overcome, this result is underpinned by good science,” says Kingham. “We hope that this work will attract further investment in fusion energy.”

Many challenges remain

Howard Wilson, director of the York Plasma Institute at the University of York in the UK, points out, however, that the result relies on being able to achieve a very high magnetic field. “We have long been aware that a high magnetic field enables compact fusion devices – the breakthrough would be in discovering how to create such high magnetic fields in the tokamak,” he says. “A compact fusion device may indeed be possible, provided one can achieve high confinement of the fuel, demonstrate efficient current drive in the plasma, exhaust the heat and particles effectively without damaging material surfaces, and create the necessary high magnetic fields.”

The work by Tokamak Energy follows an announcement late last year that the US firm Lockheed Martin plans to build a “truck-sized” compact fusion reactor by 2019 that would be capable of delivering 100 MW. However, the latest results from Tokamak Energy might not be such bad news for ITER. Kingham adds that his firm’s work means that, in principle, ITER is actually being built much larger than necessary – and so should outperform its Q target of 10.

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Your task: Take apart, decontaminate, refurbish, relocate, reassemble, realign and reinstall a 75-ton neutral beam box that will add a second beam box to the National Spherical Torus Experiment-Upgrade (NSTX-U) and double the experiment’s heating power. Oh, and while you’re at it, hoist the two-story tall box over a 22-foot wall.

Members of the “Beam Team” faced those challenges when moving the huge box from the Tokamak Fusion Test Reactor (TFTR) cell to the NSTX-U cell. The task required all the savvy of the PPPL engineers and technicians who make up the veteran team. “They’re a tight-knit group that really knows what they’re doing,” said Mike Williams, director of engineering and infrastructure and associate director of PPPL and a former member of the team himself.

The second box is one of the two major components of the upgrade that will make NSTX-U the most powerful spherical tokamak fusion facility in the world when construction is completed early next year. The new center stack that serves as the other component will double the strength and duration of the magnetic field that controls the plasma that fuels fusion reactions.

The two new components will work together hand-in-glove. The stronger magnetic field will increase the confinement time for the plasma while the second beam box performs double-duty. Its beams will raise the temperature of the plasma and will help to maintain a current in the plasma to demonstrate that future tokamaks can operate in a continuous condition known as a “steady state.” The second box is “an absolutely crucial part of the upgrade,” said Masayuki Ono, project director for the NSTX-U.

PPPL Tokamak

Work began in 2009

Work on the second beam box began in 2009 when technicians clad in protective clothing dismantled and decontaminated the box as it sat in the TFTR test cell. While the box had used radioactive tritium to heat the plasma in TFTR, no tritium will be used in NSTX-U experiments.

The decontamination took huge effort, said Tim Stevenson, who led the beam box project. Workers wearing protective garb used cloths, Windex and sprayers with deionized water to clean every part of the box by hand, and went over each part as many as 50 separate times. The cloths were then packaged and shipped to a Utah radiation-waste disposal site.

Next came the task of moving the beam box and its cleaned and refurbished components out of the TFTR area and into the NSTX-U test cell next door. But how do you get something so massive to budge?

The Beam Team solved the problem with air casters, said Ron Strykowsky, who heads the NSTX-U upgrade program. Using a ceiling crane, workers lifted the box onto the casters, which floated the load on a cushion of air just above the floor, enabling forklifts to tow it. Technicians then removed some hardware from the large doorway between the two test cells so the beam box could get through.

The doorway led to a section of the NSTX-U area that is separated from the vacuum vessel by a 22-foot shield wall — a barrier too high for the box and its lid to clear when suspended by sling from a crane. Workers surmounted the problem by first lifting the box and then the lid, which had been removed during the decontamination process. The parts cleared the wall and sailed over the vacuum vessel before coming to rest on the test cell floor. The vessel itself was wrapped in plastic to prevent contamination from any tritium that might still be in the box and the lid as they swung by overhead.

“Like rebuilding a ship in a bottle”

The beam box was now ready to be reassembled and reinstalled. But carving out room for all the parts and equipment, including power supplies, cables, and cooling water pipes, proved difficult. “There were so many conflicting demands for space that it was like rebuilding a ship in a bottle,” Stevenson said, citing a remark originally made by engineer Larry Dudek, who heads the center stack upgrade project. “There was no existing footprint,” Stevenson said. “We had to make our own footprint.”

Technicians needed to cut a port into the vacuum vessel for the beam to pass through. But the supplier-built unit that connected the box to the vessel left too much space between the unit and this new port, requiring the Welding Shop to fill in the gap. “The Welding Shop saved the port,” Stevenson said.

Still another challenge called for ensuring that the beam would enter the plasma at precisely the angle that NSTX-U specifications required. Complicating this task was the test cell’s uneven floor, which meant that the position of the box also had to be adjusted. To align the beam, engineers used measurements to derive a bull’s-eye on the inside of the vessel; technicians then used laser technology to zero in on the target. The joint effort aligned the beam to within 80 thousands of an inch of the target.

Installing power supplies

Left to complete was installation of power supplies, a task accomplished earlier this year. The job called for bringing three orange high-voltage enclosures — the source of the power — up from a basement area and into the test cell through a hatch in the floor. Taken together, the two NSTX-U beam boxes will have the capacity to put up to 18 megawatts of power into the plasma, enough to briefly light some 20,000 homes.

When asked to name the greatest challenge the project encountered, Stevenson replied, “The whole thing was fraught with challenges and difficulties. It was a monumental team effort that took a great deal of preparation. And when it was show-time, everyone showed up.”